In most Integrated Circuit (IC) processes, the active components are formed by an n-type epitaxial layer formed on a substrate of p-type.
Around the active components a p-type diffusion is formed for providing an isolation. The p-type isolation is short circuited to the substrate of the integrated circuit. The substrate must in such a case be given a potential corresponding to the lowest potential in the circuit in order to be able to provide a reverse biasing of the substrate and isolation pn-transitions in relation to islands of active components formed on the substrate.
However, even though the islands of active components are reverse biased to the substrate, there are always currents from the components to the substrate, so called substrate currents. The substrate currents can be leakage currents from reverse biased areas around the component islands or other minority charge carriers injected from, for example, a forward biased base in an npn transistor or a forward biased emitter or collector in a lateral pnp-transistor.
In circuits designed to have high voltage supplies, these substrate currents can contribute to a significant part of the total power consumption in the integrated circuit.
The potential of the substrate is usually supplied via a pin provided on the package of the integrated circuit. The pin is in turn connected to a pad on the circuit, which pad in turn is connected via a metal conductor to the isolation diffusion area to the substrate. Since the substrate voltage is distributed over the entire integrated circuit it is possible, anywhere on the integrated circuit, to attach a contact to an isolation diffusion in order to gain access to the substrate voltage.
Thus, it is not necessary to form separate metal connections for this voltage unless a very low resistance or very low noise is required in the application. In some applications the substrate itself and the isolation is made a part of the semiconductor component, for example in a so-called substrate pnp transistor, where the isolation around the transistor and the substrate is designed as a collector.
Since the substrate potential is the lowest potential in a circuit, the substrate potential together with the highest supply voltage will decide the available voltage range in the circuit.
In some applications, for example a Subscriber Line Interface Circuit (SLIC) providing a ring signal in a telephone system, it is desired to vary the output voltage from the circuit.
In the case with a SLIC providing an output ring signal, it is common to use a so called off-hook battery when the SLIC is in a speech mode and another battery, a so called ring battery, having an output voltage being higher, in absolute value, than the off-hook battery in a ringing mode.
The supply voltages in SLICs is usually lower than the ground voltage, and the potential of the substrate determines the voltage range available in the SLIC. In order to switch between the different batteries used in the different modes an integrated battery switch is provided.
In FIG. 1 such a switch S in accordance with the prior art is shown implemented in a SLIC being designed to output a ring signal. During transmission of speech, when the SLIC is in a speech mode, a voltage applied over the load RL, i.e. the telephone set of a subscriber being connected to the SLIC and the line to/from the subscriber, will be lower in absolute value than the voltage of the off-hook battery OHB.
The current from the load RL will pass a diode D2 to the off-hook battery OHB, which can have a voltage of about −50 volt and the voltage Vbat2 applied to the load will be about −50 volt.
When the SLIC is in a speech mode the switch S will be open. When it is determined that the SLIC is to output a ring signal over the load RL, i.e. when it is determined that the telephone set of the subscriber connected to the SLIC is to ring, the switch is activated and closed.
As a result, the potential Vbat2 will be the same as the voltage of the ringing battery RB, in this case −90 volt. The diode D2 will now be reverse biased and will not conduct any current. The line driving circuits A and B are now connected to the ringing voltage and can apply it over the load in order to thereby increase the voltage when delivering the ring signal.
However, in speech-mode, there is a problem of power losses associated with the technique in accordance with the prior art. Thus, the leakage current from the circuitry will flow to the substrate sbt, which has a potential corresponding to a high voltage drop and hence the leakage currents will cause a significant power loss.